Mass spectrometry (MS) with atmospheric pressure ionization techniques, such as electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), coupled with liquid chromatography (LC) has become one of the most powerful techniques for the analysis of biomolecules and pharmaceuticals. In numerous laboratories around the world, LC/MS combined technique has been a valuable implementation.
The techniques for coupling LC with MS have become diversified with increased capability of LC/MS. Desorption electrospray ionization (DESI), which was originally developed by Professor Graham Cooks et al. from Purdue University, has been introduced for direct sample ionization with little or no sample preparation. In addition to analysis of solid samples, DESI's use has been extended for direct liquid sample analysis.
Splitting eluent in an LC/MS experiment is often necessary when the mobile phase flow rate is too high for MS ionization. For instance, ESI requires an optimal sample infusion rate at a μL/min level, whereas a mobile phase flow rate for chromatographic separation using regular analytical LC columns is in the range of approximately 1-2 mL/min. Even higher flow rates (up to 9 mL/min) are needed for ultra-fast LC separation using monolithic columns. In addition, post-LC column splitting is needed when a remaining portion of LC eluent after MS detection needs to be collected for a preparative purpose.
Typically, post-LC column splitting of LC eluent can be made using a Tee splitter in which one of the split eluent streams goes to a mass spectrometer for detection and the other may go to waste or to a second detector. However, a connection capillary bridging the Tee and MS, as well as the Tee splitter itself, introduces dead volume and may cause peak broadening. Furthermore, the flow rate of the eluent flowing into the mass spectrometer is significantly reduced after splitting (e.g., the flow rate can be dropped by 100 times from 1 mL/min to 10 μL/min after splitting). Both factors contribute to a long time delay for MS detection. Therefore, using a Tee, it is difficult to collect LC analytes in the other “waste” stream because the analytes in the “waste stream” could flow out of the LC/MS apparatus prior to split eluent being detected by MS. In addition, the introduced dead volume will lead to peak broadening. Moreover, use of a post-column Tee splitter increases LC column back pressure. Thus, a new LC/MS interface that allows splitting LC eluent for both fast online MS detection and online sample collection without introducing significant dead volume and back pressure is desired.